Subscription television systems, such as cable TV, serve customers on a paid subscription basis. There are hundreds of cable TV operations reaching tens of millions of television viewers throughout the United States. A single cable TV system may have thousands of subscribers, each of which pays a monthly service charge, the amount of which depends upon how many and which television channels the subscriber pays to receive. It is necessary therefore for a cable system operator to be able to restrict access on a subscriber-by-subscriber basis to various of the TV channels broadcast within the system. Access by a subscriber to the various channels available in a cable TV system is controlled by means of respective subscriber terminal addresses and access codes sent to each subscriber terminal from a central office over the cable along with the television signals. Each subscriber location is provided by the cable operator with a special subscription TV receiver (subscriber terminal), each with a unique address, that is normally leased to the customer on a monthly basis. The receiver is owned and controlled by the cable TV operator. To prevent non-authorized access to one or more of the many TV channels being simultaneously transmitted over the cable, the cable TV operator typically uses not only subscriber terminal address and channel access codes, but also employs some kind of proprietary coding system for scrambling and descrambling the actual television pictures within each of the channels. Typically, the picture of each TV channel is individually scrambled at a central office at the sending end and then individually descrambled by a special receiver at each subscriber location. Each receiver receives from the central office coded data signals which instruct the respective receiver to descramble (or not) those TV channels for which access is authorized (or not). Thus TV channel access on a subscriber-by-subscriber basis is easily controlled electronically by the cable TV operator from the central office.
There are various scrambling and descrambling systems commercially available to cable TV operators from several manufacturers of such systems. These systems are usually highly proprietary, with certain details of the operation of each being cloaked in secrecy in so far as possible. The scrambling system sold by one manufacturer is generally not compatible with similar systems from other manufacturers. Descrambling equipment is made difficult to copy in order to deter industrial piracy of the equipment or modification thereof to obtain unauthorized access to paid subscription TV channels.
Once a cable TV operator has equipped his system with the scrambling and descrambling equipment made by a first manufacturer, it becomes difficult, if not impossible, for the operator to add equipment from a second manufacturer to that cable TV system because of the problems of compatibility of different scrambling and descrambling methods and apparatus. In effect, the cable TV operator is locked into a one vendor monopoly relation with the first manufacturer. Heretofore a cable operator who does business with one manufacturer and wants to do business with another, has had to re-equip an entire cable TV system with new scrambling and descrambling equipment. This change can be very expensive.
This lack of compatibility between different makes of scrambling and descrambling equipment has been a significant barrier to a manufacturer of new equipment in selling to the many already established cable TV systems. This is true even though the manufacturer may have better prices, produces technically superior equipment to that already in place, has a much stronger industry position, and/or offers additional products and services not available from other manufacturers.
It is desirable therefore for a manufacturer of scrambling and descrambling equipment, without having to face problems of equipment compatibility within an existing cable TV operation, to be able to supply his equipment for new subscribers to the operation as they are added side-by-side with existing subscribers who continue to use different scrambling and descrambling equipment already in place. The present invention provides an economical and highly effective solution to this need by providing a unique scrambling and descrambling system which establishes compatibility between previously installed equipment and modes of operation without the need to modify or control that equipment.